Heteroepitaxial Ge1-xSix layers on Si(100) with high Ge contents are of interest due to many applications in key technologies such as solar cells, MEMS, quantum cascade lasers and Si-based photonics1,2 including high speed modulators and photodetectors.3 However, these materials are much less developed in spite of this high impact potential in IR optical devices. In addition, they serve as virtual substrates for growth of high mobility, strained Si and Ge device channels, and are also considered as a potential pathway to monolithic integration of III-V based devices with Si technologies.4,5 The best current route to these materials is complicated and fraught with difficulties, requiring both high temperature growth of thick (>10 μm) compositionally graded films and a chemical-mechanical planarization step to relieve the misfit strain between the Ge1-xSix epilayer and Si substrate and produce a flat surface, respectively.6 
To circumvent these difficulties we have recently developed new chemical vapor deposition (CVD) heteroepitaxy to produce device quality strain-relieved Ge-rich films on Si without the need for graded compositions.7,8 Our strategy involves depositions of single source hydride precursors with direct Si—Ge bonds such as the (H3Ge)xSiH4-x (x=1-4) family of compounds. These are routinely synthesized with semiconductor grade purity via straightforward methodologies utilizing commercially available starting materials.7 Their physical and chemical properties such as high volatility and facile reactivity make them particularly useful as reagents in low temperature film growth by CVD and gas source molecular beam epitaxy (GSMBE). Our initial growth experiments have yielded single crystal films with compositions Si0.50Ge0.50, Si0.33Ge0.67, Si0.25Ge0.75 and Si0.20Ge0.80, precisely matching those of the corresponding precursors SiH3GeH3, (H3Ge)2SiH2, (H3Ge)3SiH and (H3Ge)4Si.8 Our results demonstrate that exact control of the composition, structure and strain at the atomic level can be achieved via incorporation of the entire Si—Ge framework of the precursors into the film. The resulting films are of much higher quality than those previously reported using conventional sources and grow at much lower temperatures that are compatible with CMOS technology.
Within this compositional range the Si0.50Ge0.50 semiconductor system is of particular importance because it possesses the ideal lattice dimensions to integrate fully strained Si channels exhibiting high electron mobilities into metal oxide silicon field effect transistors (MOSFETS). Thus these materials are ideally suited for immediate technological applications in high-speed devices. However, in our experiments we have observed that stoichiometric Si0.50Ge0.50, with the required device quality properties (surface planarity, low defect densities), can only be grown near a maximum temperature of ˜450° C. at a growth rate of only 0.2 nm/min which is too low to be practical for high throughput device fabrication. Negligible film growth was observed at lower temperatures via H3GeSiH3, while films with rough surfaces and high dislocation densities were obtained above ˜500° C. due to the thermal mismatch with the substrate.
The synthesis of new compounds with increased reactivities and improved properties for device fabrication relative to H3GeSiH3 would be of great value in the art.
In this study we synthesize and fully characterize the butane-like (GeH3)2(SiH2)2 (1) compound containing highly reactive —H2Si—SiH2— functionalities (rather than SiH3). The H2Si═SiH2 dimeric unit has been identified as the dominant reactive intermediate in high temperature CVD via decomposition of conventional Si-hydrides. These species lead to highly efficient growth of homoepitaxial Si.9 Precursor (1) is specifically designed to incorporate these necessary —H2Si—SiH2— building blocks and provide a new low temperature route for development of a broad range of Si-containing materials. Here we demonstrate the utility of (1) to produce device quality Si0.50Ge0.50 alloys at unprecedented low temperatures of 350° C. A 10-fold improvement in the growth rate (˜2.0 nm/min) is achieved at 350° C. relative to growth (0.2 nm/minute) at 450° C. via SiH3GeH3 leading to uniformly stressed or strain-relaxed thin films directly on Si with planar surfaces and remarkably reduced dislocation densities. En route to (1) we have produced the isobutane analog (GeH3)2SiH(SiH3) (2) and have also isolated and fully characterized the Ge-rich derivative (GeH3)2(SiH2GeH2) (3). The latter is a moderately volatile liquid, with potential applications in low temperature synthesis of Ge-rich optoelectronic alloys.